1. Field of the Invention
The invention is directed to novel thermoplastic blends of polyphenylene ether resins and high impact rubber modified polystyrenes with good resistance to environmental stress cracking together with good melt flow characteristics.
2. Brief Description of the Prior Art
Polyphenylene ether resins are high performance, naturally flame retardant, engineering thermoplastics having relatively high melt viscosities and high softening temperatures (generally in excess of 200.degree. C.). However, it has long been recognized that they have a major drawback of not being easily melt processed. To improve moldability and processability polyphenylene ether resins have been blended with polystyrene resins, particularly high impact polystyrene resins such as the rubber modified graft copolymers of styrene on butadiene-based elastomers. Such blends are disclosed, for example, by Cizek (to General Electric Co.), U.S. Pat. No. 3,383,435 (1968).
Blends of polyphenylene ethers with styrenic polymers, especially high impact polystyrene, have been found extremely useful and are major compositions of commerce. However, improvements are desired in regard to stress cracking of articles molded from these resin blends. Stress cracking is the most frequently occurring failure mode which imposes an upper limit on service life of the molded articles. This failure mode occurs when microcracks are initiated in regions of the article where the local stress exceeds a critical stress level. A microcrack can propagate and become a macroscopic failure.
It is further known that in the presence of certain chemicals, fuels, oils and solvents, the critical stress level is appreciably reduced, thus causing increased potential for stress cracking. Such induced stress cracking is called environmental stress cracking (ESC). An example is the ESC of plastic refrigerator claddings caused by the fluorochlorocarbon blowing agent residues in the polyurethane foam insulation. Another situation where ESC is a problem is in automotive applications; ESC sensitized by gasoline or oils has prevented use of a number of polyphenylene ether resins where the plastic would otherwise be useful.
Various means have been sought to overcome ESC. We have already disclosed in U.S. Pat. Nos. 3,819,761 (1974) and 3,976,725 (1976) that improved ESC resistance may be achieved by use of high intrinsic viscosity, rubber-modified polystyrenes in a polyblend with a polyphenylene ether resin. However, since such blends have inherently high viscosities, they are difficult to process.
Another method for attempting to overcome ESC is to add an alkylsulfonate, as taught by Lohmeijer (to General Electric Co.), U.S. Pat. No. 4,529,761 (1985), or an alkylsulfonate together with a vinylaromaticdiene block copolymer, as taught by de Munck et al. (to General Electric Co.), U.S. Pat. No. 4,647,594 (1987). However, the use of such an additional component adds mixing complications which are undesirable.
It is one object of the present invention to obtain polyphenylene ether resin containing molding compositions having good melt flow (good moldability) along with improved environmental stress cracking resistance, without suffering loss of the other useful and characteristic properties of polyphenylene ether-impact polystyrene blends such as good impact strength and processability.
I have now found that a combination of improved melt flow along with improved environmental stress crack resistance, and generally other desirable physical properties, can be achieved by means of polyphenylene ether-polystyrene blends wherein low intrinsic viscosity polyphenylene ether and very high intrinsic viscosity polystyrene is used to prepare the blend.